The pinhole is disposed at the outgoing-light focusing position of the object end in a conjugate focus optical system. The outgoing-light focusing position optically corresponds to the position of the conjugate focus in a light-collecting optical system. The pinhole filters off the light echo not coming from the focus of the object end to reduce the noise of light echo in the optical system and obtain an image more delicate than that obtained with an optical system without pinhole. Since the position of the pinhole is fixed, the calibration of the focusing position of the signal beam on the plane of the pinhole is normally achieved by adjusting the focusing lens of the light-collecting optical system. That is, the position of the focusing lens on the plane of the pinhole can only be adjusted in the X-Y directions but not in the Z direction. The X-Y directions are perpendicular to each other, and the Z-direction is perpendicular to the pinhole plane. The Z-direction calibration cannot be achieved because the distance between the pinhole and the detection element is fixed.